The present invention relates to a capacitor having a textured pellet and a method for making same.
Solid tantalum capacitors and solid niobium capacitors use as anodes porous pellets which are compressed from fine tantalum or niobium powder. The pellets are first compressed and then they are sintered in a vacuum. The porous structure of the anode provides a high surface area and thereby high specific charge (CV) of the finished capacitor (where CV refers to capacitance and rated voltage). Generally tantalum and niobium pellets contain a lead wire made of the same metal as the compressed powder. The lead wire is attached to the pellet either by welding after the pellet has been sintered, or by embedding in the pellet before the pellet is pressed and sintered.
Prior art efforts have been made to increase the surface area of the pellet by fluting the exterior surface of a cylindrical pellet, or by forming elongated bores or channels in the interior of the pellet. In these prior art efforts the press dye forming the pellet determines the shape and configuration of the pellet, and hence of the anode. A large part of the available pellet volume is lost in these prior art efforts, and the ability to coat the outside surface of these pellets was not enhanced.
FIG. 8 illustrates one of these prior art devices which includes a cylindrical body 10 having elongated flutes 12 extending along the length thereof. This capacitor removes a great deal of the volume of the body 10 and does not significantly enhance the ability of the manganese oxide layer to adhere to the outer surface of the body 10.
FIG. 10 shows another prior art body 14 having two or more elongated bores 16 extending therethrough. This increases the surface area of the body, but removes a significant volume, thereby reducing the overall capacitance of the body.
Therefore, a primary object of the present invention is the provision of an improved capacitor having a textured pellet and method for making same.
A further object of the present invention is the provision of a capacitor having a textured pellet which increases the adhesive characteristic for the coating which is coated on the outside surface of the pellet.
A further object of the present invention is the provision of an improved capacitor having a textured pellet which improves the consistency of the ESR distribution from capacitor to capacitor and results in reduced failures related to any penetration of conductive coatings through cracks or voids.
A further object of the present invention is the provision of an improved capacitor which lowers ESR of the capacitors at mid to high frequency and increases the adhesive strength of the MnO.sub.2 coating on the exterior of the pellet significantly.